Within the last few years, apparatus has been devised for hot briquetting of metallic particles such as cast iron borings produced when cast iron is machined (such borings are often known as "chips"). In the apparatus the particles are fed to a furnace where they are heated to a temperature which they are plastic but not to the temperature which they melt, the hot particles being discharged from the furnace to a press where they are compacted into briquettes. In the case of cast iron particles, the briquettes thus produced are sold to a manufacturer of cast iron and are normally re-melted in a cupola or an induction or arc furnace to produce foundry castings.
In the furnace any moisture which the particles contain is vapourised and is driven off as steam. Further, any oil which the particles contain also vapourises and is burnt or driven off. The percentage by weight of oil may vary enormously and the combustion of vapourised oil does, of course, require some of the oxygen in the atmosphere in the furnace. The atmosphere in the furnace is desirably a reducing atmosphere in order to prevent oxidisation of the carbon, silicon and manganese in the particles. Attempts have been made to control the air flow to the furnace to ensure that at all times the furnace atmosphere is a reducing atmosphere but difficulties have been encountered in achieving adequate control and some carbon oxidation usually occurs.
In one known apparatus the particles are fed to the lower chamber of a furnace which lower chamber could be decribed as a drier. The furnace contains an upper chamber where combustion of any oil in particles takes place and from which gases are exhausted into a gas stack while the hot particles fall from the lower chamber of the furnace through a discharge chute into the briquetting press.
In this known prior art apparatus there is a manual control to set the fuel to air ratio for high oil and low oil conditions. For the "high oil" condition, that is to say the particles have a high oil content, the air supply is set so as to give a relatively high air to fuel ratio for the burners so that there is sufficient air available to permit combustion of the oil on the particles, i.e. the excess air condition. In the "low oil" condition, it is assumed that the particles have no oil at all and the air to fuel ratio is set to be the stoichiometric condition, that is to say enough air is supplied to the furnace for combustion of all the fuel supplied to the furnace but no excess air is supplied.
Further, with this known apparatus the temperature control within the furnace is carried out at various zones. One zone is the particle inlet to the lower chamber where the burner is located. A second zone is in the lower chamber at the opposite end to the particle inlet where there are two burners jointly controlled.
A third zone is which temperature control is carried out is in the upper chamber where there is provided a further pair of adjacent and cross fired burners approximately in line with the inlet of gases to the upper chamber from the lower chamber which is near to the end of the lower chamber where the particles are fed into that lower chamber. The air flow to the intermediate portion (where there are no burners) of the upper chamber is also temperature controlled; this air flow is jet or secondary air.
From the opposite end of the upper chamber, that is to say, the end of the upper chamber opposite the particle inlet, a gas stack extends and at the base of the gas stack there is a damper to control the ingress of ambient air. There are thus five zones of temperature control, that is to say the first, second and third burner zones, the air flow to the intermediate portion of the upper chamber and the stack. Each of these zones is separately controlled, that is to say adjacent to each of the burners, in the intermediate portion of the upper chamber, and in the stack, there is provided a thermocouple. Each of the burner thermocouples has its output set to control the flow of combustion air to the associated burner(s). In the event that the thermocouple senses a temperature below that desired, the thermocouple controls an associated motorised valve to increase the combustion air. A pressure sensing line is connected between the combustion air flow and the associated fuel inlet of that burner zone so that an increase in combustion air is accompanied by a corresponding increase in fuel, maintaining the fuel to air ratio in that zone constant. The thermocouple is arranged to control the air flow to the intermediate portion of the upper chamber so that a rise in temperature leads to the air flow being increased to reduce the temperature and correspondingly a reverse operation takes place in the event that the thermocouple senses a drop in temperature.
The thermocouple in the stack is set to control the position of the damper at the base of the stack.
In practice, the burners in the upper chamber of the furnace are often found to be unnecessary and is manually switched off. Further, in practice, the thermocouple in the stack always sets the damper at the base of the stack to the position in which it is totally closed.
The "high oil" and "low oil" controls are associated with all three burner zones and when set ensure that the air to fuel ratio for the three burner zones remain constant.
The described apparatus for hot briquetting is intended to maintain a reducing atmosphere in the furnace but this, in practice, is not achieved. U.S. Pat. Nos. 4,133,635 and 4,260,373 refer to apparatus of this type.